System and method for fase imaging in magnetic resonance imaging

ABSTRACT

A system and method for magnetic resonance imaging is provided. The method includes generating a main magnetic field through a region of interest (ROI), applying a slice selection gradient to an slice of the ROI, applying a plurality of RF pulses to the slice to generate a plurality of echoes, applying a first encoding gradient and a second encoding gradient on the echoes, and generating MR images based on the echoes.

TECHNICAL FIELD

The present disclosure generally relates to magnetic resonance imaging(MRI), and more particularly, to a system and method for fast imaging inMRI.

BACKGROUND

Magnetic Resonance Imaging (MRI) is a widely used medical techniquewhich produces images of a region of interest (ROI) by exploiting apowerful magnetic field and radio frequency (RF) techniques. When an MRIscan is performed on a subject, various RF pulse sequences may betransmitted through an ROI of the subject. The time of transmitting theRF pulse sequences and receiving MR signals determines the acquisitiontime (TA) of the MRI scan. Given the high power of those pulsesequences, long time exposure under the RF pulse sequences for a subjectmay cause physical damages to the subject. Therefore, it is desirable toreduce acquisition time for a concern of safety.

SUMMARY

In a first aspect of the present disclosure, a method for generating amagnetic resonance (MR) image is provided. The method may includegenerating a main magnetic field through an region of interest (ROI);applying a slice selection gradient to an slice of the ROI; applying aplurality of RF pulses to the slice of the ROI to generate a pluralityof echoes; applying a first encoding gradient in a first direction andsimultaneously applying a second encoding gradient in a second directionon each echo, wherein the amplitude of the first encoding gradient whenacquiring the center region of a k-space being lower than that of thefirst encoding gradient when acquiring the peripheral region of thek-space; generating a plurality of undersampled k-space data sets basedon the encoding gradients; and generating an MR image by applying atleast one image reconstruction method to the undersampled k-space datasets.

In a second aspect of the present disclosure, provided herein is amagnetic resonance imaging (MRI) system, the MRI system may comprise anMRI scanner, a control unit, and a processing unit. The MRI scanner maycomprise a main magnet field generator configured to generate a mainmagnetic field through a region of interest (ROI), a gradient fieldgenerator configured to apply a slice selection gradient to a slice ofthe ROI, to generate a first encoding gradient in a first direction ,and to generate a second encoding gradient in a second direction, and anRF transmit/receive unit configured to transmit a plurality of RF pulsesto the slice of the ROI to generate a plurality of echoes. The gradientmagnet field generator may be configured to apply the first encodinggradient in the first direction and the second encoding gradient in thesecond direction simultaneously on each echo, the amplitude of the firstencoding gradient when acquiring the center region of the k-space may belower than that of the first encoding gradient when acquiring theperipheral region of the k-space.

In some embodiments, the RF pulses may comprise fast spin echo (FSE).

In some embodiments, the waveform of the first encoding gradient in thefirst direction may include three steady phases and two phases oftransition.

In some embodiments, the waveform of the first encoding gradient in thefirst direction may comprise part of a function having a smoothvariation. In some embodiments, the function having a smooth variationmay comprise a Gaussian function or a harmonic function.

In some embodiments, the second encoding gradient in the seconddirection may comprise an oscillating waveform. In some embodiments, thewaveform may oscillate in a periodic way an aperiodic way.

In some embodiments, the applying a first encoding gradient in the firstdirection may comprise: applying at least two different encodinggradients for two different echoes, respectively.

In some embodiments, the first encoding gradient in the first directionmay include at least one of a dephasing gradient and rephasing gradient.

In some embodiments, the second encoding gradient in the seconddirection may include at least one of a dephasing gradient and arephasing gradient.

In some embodiments, the distribution density of an undersampled k-spacedata set in the center region of the k-space may be larger than thedistribution density of an undersampled k-space data set in theperipheral region of the k-space.

In some embodiments, the image reconstruction method may comprise atleast one of compressed sensing, parallel imaging, or partial Fourierreconstruction.

Additional features will be set forth in part in the description whichfollows, and in part will become apparent to those skilled in the artupon examination of the following and the accompanying drawings or maybe learned by production or operation of the examples. The features ofthe present disclosure may be realized and attained by practice or useof various aspects of the methodologies, instrumentalities andcombinations set forth in the detailed examples discussed below.

BRIEF DESCRIPTION OF THE DRAWINGS

The methods, systems, and/or programming described herein are furtherdescribed in terms of exemplary embodiments. These exemplary embodimentsare described in detail with reference to the drawings. Theseembodiments are non-limiting exemplary embodiments, in which likereference numerals represent similar structures throughout the severalviews of the drawings, and wherein:

FIG. 1 is a block diagram of a magnetic resonance imaging (MRI) systemaccording to some embodiments of the present disclosure;

FIG. 2 is a block diagram of an MRI system according to some embodimentsof the present disclosure;

FIG. 3 is a flowchart of an MR scan that may be performed according tosome embodiments of the present disclosure;

FIG. 4 is a graph illustrating exemplary T1-relaxation and exemplaryT2-relaxation in a magnetic resonance imaging process according to someembodiments of the present disclosure;

FIG. 5 is a graph illustrating an exemplary echo train for magneticresonance imaging according to some embodiments of the presentdisclosure;

FIG. 6 is a block diagram illustrating a processing unit according tosome embodiments of the present disclosure;

FIG.7 is a flowchart illustrating a process of a processing unitaccording to some embodiments of the present disclosure;

FIG. 8 is a block diagram of an image processing module according tosome embodiments of the present disclosure;

FIG. 9 is a flowchart illustrating an image processing according to someembodiments of the present disclosure;

FIG. 10 illustrates a flowchart of k-space sampling according to someembodiments of the present disclosure;

FIG. 11 shows a diagram of a 2D FSE pulse sequence for a singlerepetition time (TR) according to some embodiments of the presentdisclosure;

FIGS. 12A-12F show exemplary x-direction encoding gradients according tosome embodiments of the present disclosure;

FIGS. 13A-13F show exemplary y-direction encoding gradients according tosome embodiments of the present disclosure;

FIG. 14 is an exemplary diagram illustrating the distribution ofbaselines of the k-space trajectory according to some embodiments of thepresent disclosure;

FIG. 15 is an exemplary diagram of k-space sampling according to someembodiments of the present disclosure;

FIG. 16 illustrates an exemplary diagram of k-space sampling for waterphantom according to some embodiments of the present disclosure;

FIGS. 17A-17C illustrate MR images for resolution water phantomaccording to some embodiments of the present disclosure;

FIGS. 18A-18C illustrate MR images for Shepp Logan water phantomaccording to some embodiments of the present disclosure;

FIG. 19 illustrates an exemplary diagram of k-space sampling for a kneeaccording to some embodiments of the present disclosure; and

FIGS. 20A-20C illustrate MR images for knees according to someembodiments of the present disclosure.

DETAILED DESCRIPTION

The following description is presented to enable any person skilled inthe art to make and use the invention, and is provided in the context ofa particular application and its requirements. Various modifications tothe disclosed embodiments will be readily apparent to those skilled inthe art, and the general principles defined herein may be applied toother embodiments and applications without departing from the spirit andscope of the present invention. Thus, the present invention is notlimited to the embodiments shown, but is to be accorded the widest scopeconsistent with the claims.

It will be understood that when a module or unit is referred to as being“on”, “connected to” or “coupled to” another module or unit, it may bedirectly on, connected or coupled to the other module or unit orintervening module or unit may be present. In contrast, when a module orunit is referred to as being “directly on,” “directly connected to” or“directly coupled to” another module or unit, there may be nointervening module or unit present. As used herein, the term “and/or”includes any and all combinations of one or more of the associatedlisted items.

The terminology used herein is for the purpose of describing particularexample embodiments only and is not intended to be limiting. As usedherein, the singular forms “a,” “an,” and “the” may be intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprise”and/or “comprising,” when used in this specification, specify thepresence of stated features, integers, steps, operations, elements,and/or components, but do not preclude the presence or addition of oneor more other features, integers, steps, operations, elements,components, and/or groups thereof.

The present disclosure generally relates to magnetic resonance imaging(MRI), and more particularly, to a system and method for fast imaging inMRI. An MRI image may be generated by manipulating a virtual spacecalled the k-space. The term “k-space,” as used herein, may refer to anarray of numbers (a matrix) representing spatial frequencies in an MRimage. In some embodiments, k-space may be the 2D or 3D Fouriertransform of an MR image. The way of manipulating the k-space, referredas k-space sampling, may affect acquisition time (TA). As used herein,the term “acquisition time” may refer to the time to acquire signals ofthe whole pulse sequence. For example, the term “acquisition time” mayrefer to the time to obtain the whole k-space data sets from startingfilling the k-space. Traditionally, two k-space sampling methods,Cartesian sampling and non-Cartesian sampling, are provided tomanipulate k-space. In Cartesian sampling, k-space trajectories arestraight lines, while in non-Cartesian sampling, such as radial samplingor spiral sampling, k-space trajectories may be longer than those ofCartesian sampling.

Cartesian sampling may depend on its encoding gradient. Reducingacquisition time in Cartesian sampling may be realized by reducing phaseencoding steps and/or increasing analog to digital converter (ADC)bandwidth. However, in Cartesian scanning, the bandwidth of the ADC maybe subject to the restriction of the desired spatial resolution and thesize of a desired field of view (FOV). In non-Cartesian sampling, asspatial sampling may be performed in multiple dimensions simultaneously,a k-space trajectory may be relatively longer than that of Cartesiansampling.

When non-Cartesian sampling such as radial sampling and spiral sampling,are applied in 2D FSE, artifacts may be generated. On the one hand, inconventional non-Cartesian sampling such as radial sampling and spiralsampling, echoes with different intensity may be filled into k-space andthe resulting image may contain streaking artifacts due to, for example,T2-relaxation. On the other hand, in conventional non-Cartesian samplingsuch as radial sampling and spiral sampling, trajectories may passthrough the center region of the k-space. So these trajectories may haveequal contribution to the reconstructed image, which may make adjustmentof the image contrast difficult.

To reduce data acquisition time, MRI data are often intentionallyundersampled. This may, however, lead to reduced SNR (Signal to NoiseRatio) and image degradation.

Non-Cartesian sampling may have advantages over Cartesian sampling as itmay have longer k-space trajectories, and more data may be sampled whenone echo is acquired. To timely receive the sampled data, the bandwidthof ADC may need to be increased. However, the bandwidth may be subjectto limitations of some physical properties including, for example, SNR.

Consequently, it is desirable to develop a system and method capable ofreducing acquisition time in 2D FSE using non-Cartesian sampling aloneor in combination with some image reconstruction methods.

FIG. 1 is a block diagram of a magnetic resonance imaging systemaccording to some embodiments of the present disclosure. As illustrated,an MRI system 100 may include an MRI scanner 110, a control unit 120, aprocessing unit 130, and a display unit 140. The MRI scanner 110 mayinclude a gradient magnet field generator 111, a radio frequency (RF)transmit/receive unit 112, and a main magnet field generator 113. Themain magnet field generator 113 may create a static magnetic field B0during an MRI process. The main magnet may be of various typesincluding, for example, a permanent magnet, a superconductingelectromagnet, a resistive electromagnet, etc. The gradient magnet fieldgenerator 111 may generate magnet field gradients to the main magnetfield B0 in the x, y, and/or z directions. The gradient magnet field mayencode the spatial information of a subject located in the MRI scanner110.

The RF transmit/receive unit 112 may include RF transmitting coilsand/or receiving coils. These RF coils may transmit RF signals to orreceive MR signals from a region of interest (ROI). In some embodiments,the receiving coil and the transmitting coil may be the same one, orthey may be different. In some embodiments, the function, size, type,geometry, position, amount, and/or magnitude of the gradient magnetfield generator 111 and/or the main magnet field generator 113 and/or ofthe RF transmit/receive unit 112 may be determined or changed accordingto one or more specific conditions. For example, according to thedifference in function and size, the RF coils may be classified asvolume coils and local coils. In some embodiments of the presentdisclosure, the volume coils may include birdcage coils, transverseelectromagnetic coils, surface coils, saddle coils, etc. In someembodiments of the present disclosure, the local coils may includebirdcage coils, solenoid coils, saddle coils, flexible coils, etc. Insome embodiments, a multi-channel array coil having a plurality of coilelements (sometimes called phased array coils) may be employed in thepresent disclosure to receive MR signals. Specifically, themulti-channel array coil may be used to perform parallel imaging bysimultaneously receiving MR signals in each channel.

The control unit 120 may control the gradient magnet field generator 111and/or the main magnet field generator 113 and/or the RFtransmit/receive unit 112 of the MRI scanner 110, the processing unit130, and/or the display unit 140. The control unit 120 may receiveinformation from or send information to the MRI scanner 110, theprocessing 130, and/or the display unit 140. According to someembodiments of the present disclosure, the control unit 120 may receivecommands from the display unit 140 provided by, e.g., a user, and adjustthe gradient magnet field generator 111 and/or the main magnet fieldgenerator 113 and/or RF transmit/receive unit 112 to take images of anROI according to the received commands.

In some embodiments, depending on the type of magnet used in the mainmagnet generated by the main magnet field generator 113, the controlunit 120 may provide certain control signals to the gradient magnetfield generator 111 and/or the main magnet field generator 113 tocontrol various parameters of the main magnetic field that is generatedby the main magnet field generator 113. In some embodiments, the controlunit 120 may provide instructions for the gradient magnet fieldgenerator 111 and/or the main magnet field generator 113 to generate aparticular gradient waveform sequence. In some embodiments, the controlunit 120 may provide instructions for the RF transmit/receive unit 112to generate a particular pulse sequence and/or receive the MR signalfrom the ROI. In some embodiments, the control unit 120 may providetiming information to the processing unit 130, including the length ofdata acquisition, the type of k-space data acquisition that is used, orthe like, for sampling data from the RF transmit/receive unit 112. Insome embodiments, the control unit 120 may provide reconstructioninformation to the processing unit 130 to transform k-space data intoimages.

The processing unit 130 may process different kinds of informationreceived from different units. For further understanding the presentdisclosure, several examples are given below, but the examples do notlimit the scope of the present disclosure. According to some embodimentsof the present disclosure, for example, the processing unit 130 mayacquire data from the RF transmit/receive unit to fill the k-spaceaccording to the commands or instructions from the control unit 120. Theprocessing unit 130 may also receive information to transform k-spacedata into frequency-domain data, which may be a two-dimensional (2-D) orthree-dimensional (3-D) data set. As used herein, the term“frequency-domain data” may refer to the data displayed in the frequencydomain in which the feature of how much of a signal lies within eachgiven frequency band over a range of frequencies may be displayed. Theprocessing unit 130 may also receive the information to map or transformthe frequency-domain data into optical data. For example, in amonochrome display, the frequency-domain data may be mapped ortransformed into the luminance values of pixels. In a color display,frequency-domain data may be mapped or transformed into the luminanceand false-color values of pixels. The process unit 130 may also receivethe information to transform the optical data into signals, which may bedisplayed as the image viewed by a user. In some embodiments, theprocessing unit 130 may process data provided by a user or an operatorvia the display unit 140 and transform the data into specific commandsor instructions (for example, computer-readable commands orinstructions), and supply the commands or instructions to the controlunit 120.

The display unit 140 may receive input and/or display outputinformation. The input and/or output information may include programs,software, algorithms, data, text, number, images, voice, or the like, orany combination thereof. For example, a user or an operator may inputsome initial parameters or conditions to initiate a scan. As anotherexample, some information may be imported from external resource, suchas a floppy disk, a hard disk, a wireless terminal, or the like, or anycombination thereof.

It should be noted that the above description of the MRI system 100 ismerely provided for the purposes of illustration, and not intended tolimit the scope of the present disclosure. For persons having ordinaryskills in the art, multiple variations and modifications may be madeunder the teachings of the present disclosure. For example, the assemblyand/or function of the MRI system 100 may be varied or changed accordingto specific implementation scenarios. Merely by way of example, someother components may be added into the MRI system 100, such as a subjectpositioning unit, a gradient amplifier unit, and other devices or units.Note that the MRI system may be a traditional or a single-modalitymedical system, or a multi-modality system including, e.g., a positronemission tomography-magnetic resonance imaging (PET-MRI) system, acomputed tomography-magnetic resonance imaging (CT-MRI) system, a remotemedical MRI system, and others, etc. However, those variations andmodifications do not depart from the scope of the present disclosure.

FIG. 2 is a block diagram of the MRI system 100 according to someembodiments of the present disclosure. As shown in the figure, the mainfield and shim coils 201 may generate a main magnetic field that may beapplied to an object exposed inside the field. The main field and shimcoils 201 may also control the homogeneity of the generated main field.Gradient coils 202 may be located inside the main field and shim coils201. The gradient coils 202 may generate a second magnetic field orreferred to as a gradient field. The gradient coils 202 may distort themain field generated by the main field and shim coils 201 so that themagnetic orientations of the protons of an object may vary as a functionof their positions inside the gradient field. The gradient coils 202 mayinclude X coils, Y coils, and/or Z coils (not shown in the figure). Insome embodiments, the Z coils may be designed based on circular(Maxwell) coils, while the X coils and the Y coils may be designed onthe basis of the saddle (Golay) coil configuration. The three sets ofcoils may generate three different magnetic fields that are used forposition encoding. The gradient coils 202 may allow spatial encoding ofMR signals for image construction. The gradient coils 202 may beconnected with one or more of an X gradient amplifier 204, a Y gradientamplifier 205, or a Z gradient amplifier 206. One or more of the threeamplifiers may be connected to a waveform generator 216. The waveformgenerator 216 may generate gradient waveforms that are applied to the Xgradient amplifier 204, the Y gradient amplifier 204, and/or the Zgradient amplifier 204. An amplifier may amplify a waveform. Anamplified waveform may be applied to one of the coils in the gradientcoils 202 to generate a magnetic field in the x-axis, the y-axis, or thez-axis, respectively. The gradient coils 202 may be designed for eithera close-bore MRI scanner or an open-bore MRI scanner. In some instances,all three sets of coils of the gradient coils 202 may be energized andthree gradient fields may be generated thereby. In some embodiments ofthe present disclosure, the X coils and Y coils may be energized togenerate the gradient fields in the x-direction and the y-direction.

RF (radio frequency) coils 203 may generate a third magnetic field thatis utilized to generate MR signals for image construction. In someinstances, the RF coils 203 may include a transmitting coil and areceiving coil. In some embodiments, the RF coils 203 may be inconnection with RF electronics 209 that may be configured or used as oneor more integrated circuits (ICs) functioning as a waveform transmitterand/or a waveform receiver. The RF electronics 209 may be connected withan RF amplifier 207 and an analog-to-digital converter (ADC) 208. Thewaveform generator 216 may generate an RF signal. The RF signal may befirst amplified by the RF amplifier 207, processed by the RF electronics209, and applied on the RF coils 203 to generate a third magnetic field,in addition to the magnetic fields generated by, e.g., the main fieldand shim coils 201 and the gradient coils 202. In some embodiments ofthe present disclosure, the waveform generator 201 may generate a seriesof RF waveforms periodically or aperiodically. For instance, thewaveform generator 216 may generate an excitation RF pulse with a flipangle of 90° and multiple refocusing RF pulses with a flip angle of180°. Note that the excitation RF pulse may have a flip angle other than90°, e.g., any magnitude ranging from 0° to 180°. An excitation RF pulsewith a flip angle of 90° is mentioned elsewhere in the presentdisclosure for illustration purposes, and is not intended to limit thescope of the present disclosure.

As described elsewhere in the present disclosure, the flip angle of arefocusing RF pulse may be of a value other than 180°. Furthermore, thewaveform generator 216 may generate a series of RF waveformsperiodically or aperiodically. For instance, the waveform generator 216may generate an excitation RF pulse with a flip angle of 90° andmultiple refocusing RF pulses with same flip angles or variable flipangles. The flip angle of the excitation RF pulse may be variable aswell. The excitation RF pulse may be utilized to generate the thirdmagnetic field, and with the application of one or more refocusing RFpulses, one or more MR signals may be generated. For instance, an echotrain with multiple echoes may be generated. The echo train length (ETL)may be either fixed or variable. For instance, for a same tissue to beimaged, ETL may be fixed. For different tissues, ETL may be variable.Furthermore, even for a same tissue, ETL may be variable. The echo trainmay be received by the receiving coils of the RF coils 203. Then theecho train may be sent to the RF electronics 209, and transmitted to theADC 208 for digitization. The echo train may be demodulated and filteredin the electronics 209. Subsequently, the echo train may be processed byan image processor 211, e.g., with the assistance of the CPU 213, togenerate one or more images. A console 214 may communicate through alink with the CPU 213 and allow one or more operators to control theproduction and/or display of images on image display 212. The console214 may include an input device, a control panel (not shown in thefigure), etc. The input device may be a keyboard, a touch screen, amouse, a remote controller, or the like, or any combination thereof.

The CPU 213 may control the production of the waveforms in the waveformgenerator 216, and the production of images in the image processor 211.The CPU 213 may be a central processing unit (CPU), anapplication-specific integrated circuit (ASIC), an application-specificinstruction-set processor (ASIP), a graphics processing unit (GPU), aphysics processing unit (PPU), a microcontroller unit, a digital signalprocessor (DSP), a field programmable gate array (FPGA), an ARM, or thelike, or any combination thereof.

The data storage 215 may store received MR signals. When an MRI scan iscompleted and the whole data of a scanned object (e.g., a tissue or aspecific part of a body) is acquired. A Fourier transform of the datamay be performed by, without limitation to, the CPU 213, the imageprocessor 211, or the like, or any combination thereof. After thetransform is completed, one or more desired images may be generated. Theimages may be stored in the data storage 215. The images may be furtherconveyed to the image display 212 for display. A shim control 210 may beutilized to control the homogeneity of the main magnetic field generatedby the main field and shim coils 201.

In some embodiments of the present disclosure, an improved or optimizedflip angle schedule may be acquired according to one or more criteriadescribed elsewhere in the present disclosure. A flip angle schedule mayinclude a group of flip angles of refocusing RF pulses. The calculationof flip angles may be performed by the CPU 213. The refocusing RF pulsesmay be divided into a certain number of phases. Each phase may includeone or more refocusing RF pulses. The flip angle(s) of refocusing RFpulse(s) of each phase may be calculated in accordance with one or moreequations or functions. A signal evolution may be produced on the basisof the calculated flip angles of the refocusing RF pulses.

It should be noted that the above description of the MRI system ismerely provided for the purposes of illustration, and not intended tolimit the scope of the present disclosure. For persons having ordinaryskills in the art, multiple variations and modifications may be madeunder the teaching of the present invention. However, those variationsand modifications do not depart from the scope of the presentdisclosure.

FIG. 3 depicts a flowchart of an MR scan that may be performed accordingto some embodiments of the present disclosure. In step 301, one or moreprotocols may be selected. A protocol may be designed for one or moretissues to be imaged, diseases, and/or clinical scenarios. A protocolmay contain a certain number of pulse sequences oriented in differentplanes and/or with different parameters. The pulse sequences may includespin echo sequences, gradient echo sequences, diffusion sequences,inversion recovery sequences, or the like, or any combination thereof.For instance, the spin echo sequences may include fast spin echo (FSE),turbo spin echo (TSE), rapid acquisition with relaxation enhancement(RARE), half-Fourier acquisition single-shot turbo spin-echo (HASTE),turbo gradient spin echo (TGSE), or the like, or any combinationthereof. When an MR scan is to be conducted, an operator may select aprotocol for the scan. For example, for a cranial scan, the operator mayselect any one of the protocols called “Routine Adult Brain,” “MRAngiogram Circle of Willis,” and many others. These protocols describedabove or other protocols may be stored in the data storage 215 asdiscussed in FIG. 2, or other storage devices (e.g., an external storagedevice or server accessible by the MR system 100).

Parameters may be set in step 302. The parameters may be set via theconsole 214 through a user interface that may be displayed on, e.g., theimage display 212 as specified in FIG. 2. The parameters may includeimage contrast and/or ratio, an ROI, slice thickness, an imaging type(e.g., T1 weighted imaging, T2 weighted imaging, proton density weightedimaging, etc.), T1, T2, a spin echo type (spin echo, fast spin echo(FSE), fast recovery FSE, single shot FSE, gradient recalled echo, fastimaging with stead-state procession, and so on), a flip angle value,acquisition time (TA), echo time (TE), repetition time (TR), echo trainlength (ETL), the number of phases, the number of excitations (NEX),inversion time, bandwidth (e.g., RF receiver bandwidth, RF transmitterbandwidth, etc.), or the like, or any combination thereof.

According to some embodiments of the present disclosure, the term“phase” may refer to a segment, section, part or fragment of a series offlip angles (or a flip angle schedule) corresponding to an echo traindivided according to some principles. The number of phase(s) and/or thenumber of echo(es) in each phase may depend on specific conditions. Insome embodiments, an echo train may be divided into several phasesaccording to considerations including, e.g., the characteristics of areference signal schedule, a desired signal evolution, etc. Merely byway of example, the reference signal schedule of an echo train may bedivided into three segments, regardless of what their values are or howtheir trends vary (e.g. firstly exponential decay, secondly essentiallyflat, and lastly exponential decay again), then the echo train may bedivided into three phases accordingly. In some embodiments, thereference signal schedule may lack obvious characteristics on the basisof which to divide it into different phases. For example, only one orseveral specific echo(es) associated with resultant signal(s) ofinterest need to be paid attention to. For example, it is desired thatthe signals corresponding to two echoes meet one or more thresholds; theecho train may belong to a single phase so that the two echoes ofinterest are located in the same phase; the echo train may be dividedinto two or more phases, and the two echoes of interest may be locatedin a same phase or different phases. In some embodiments, there may beno reference signal schedule at all, and the number of phase(s) and/orthe number of echo(es) in each phase may be determined based on, e.g., arandom division, an equal division, a certain rule, or the like, or anycombination thereof. The certain rule may include Arithmeticprogression, Geometric progression, Cauchy sequence, Farey sequence,look-and-say sequence, or the like, or a variation thereof, or anycombination thereof.

It should be noted that the above embodiments are for illustrationpurposes and not intended to limit the scope of the present disclosure.The determination of the number and length of the phase(s) may bevariable, changeable, or adjustable based on the spirits of the presentdisclosure. For example, the number of phases in an echo train may beone, two, three, or more, or equal to the number of echoes. In someembodiments, several echoes may be located in one phase, and theremaining echoes belong to one or more other phases or are not assignedto a phase at all. However, those variations and modifications do notdepart from the scope of the present disclosure.

Preparation for the MR scan may be performed in step 303. Thepreparation may include placing an object, e.g., a selected portion ofan ROI, within the scanning area, setting the scanning range, tuning andmatching shimming coils, adjusting a center frequency, adjustingtransmitter attenuation/gain, adjusting signal receptionattenuation/gain, setting dummy cycles, or the like, or any combinationthereof.

The selected portion of an ROI may be scanned in step 304. The scanningmay include localizer scans, calibration scans for parallel imaging,automatic pre-scan, or the like, or any combination thereof. Forinstance, the localizer scans may produce localizer images of lowresolution and a large field of view (FOV). Such localizer images may beutilized in subsequent steps. In this step, one or more pulse sequencesincluding, for example, an excitation RF pulse and a series ofrefocusing RF pulses, may be applied on the selected portion. The flipangles of the refocusing RF pulses may be either fixed or variable. Insome embodiments of the present disclosure, the flip angles are not setin step 302 manually. Instead, the flip angles may be calculatedautomatically and an optimization procedure may be performed for thecalculation of the flip angles until a desired signal evolution isachieved.

Generated MR signals may be received in step 305. Step 305 may beperformed by the RF coils 203 as described in FIG. 2. The MR signals maycorrespond to one or more echo trains, or the like. It should be notedthat step 305 and step 306 may be repeated until sufficient data togenerate an image is acquired or an image is generated. One or moreoperations may be performed on the MR signals to produce images of theselected portion. The operations may include frequency encoding, phaseencoding, reconstruction, or the like, or any combination thereof. TheMR signals are sampled depending on the types of the gradient and RFwaveforms. Exemplary image reconstruction methods may include parallelimaging, Fourier reconstruction, constrained image reconstruction,compressed sensing, or the like, or a variation thereof, or anycombination thereof. As for dimensions, the Fourier transformation mayinclude 1-dimensional (1D) Fourier transformation, 2-dimensional (2D)Fourier transformation, 3-dimensional (3D) Fourier transformation. Asfor types, the Fourier transformation may include discrete Fouriertransformation, inverse Fourier transformation, fast Fouriertransformation (FFT), non-uniform fast Fourier transformation (NUFFT),partial Fourier transformation, or the like, or any combination thereof.Exemplary algorithms of partial Fourier transformation may include zerofilling, homodyne processing, iterative homodyne processing, or thelike, or any combination thereof. Exemplary algorithms of parallelimaging may include simultaneous acquisition of spatial harmonics(SMASH), AUTO-SMASH, VD-AUTO-SMASH, sensitivity encoding (SENSE),generalized autocalibrating partially parallel acquisitions (GRAPPA), orthe like, or any combination thereof. In step 306, one or more images ofthe selected portion may be produced. The images may be displayed on,e.g., the image display 212 (shown in FIG. 2), or other display devices(e.g., an external display device).

It should be noted that the flowchart described above is provided forthe purposes of illustration, not intended to limit the scope of thepresent disclosure. For persons having ordinary skills in the art,multiple variations and modifications may be reduced to practice in thelight of the present disclosure. However, those variations andmodifications do not depart from the scope of the present disclosure.For instance, step 301, step 302, and step 303 may be performedsequentially at an order other than that described above in connectionwith FIG. 3. Alternatively, step 301, step 302, and step 303 may beperformed concurrently.

MRI is a non-invasive imaging technique that may use a powerful mainmagnet field to align the nucleus spins in a subject (or a portionthereof). When the subject is exposed in a magnetic field (main magnetfield B0), the nucleus spins of the subject tend to align with field B0,but may still precess at the Larmor frequency. The overall motion of thenucleus spins in the subject, subject to field B0, may be simplified asnet magnetization (M) that is the averaged sum of many individualnucleus spins. The net magnetization M may be broken down into alongitudinal component (along the z-axis, aligned with field B0), and atransverse component (within the XY plane). With the effect of mainmagnet field B0, M may constitute a longitudinal magnetization vector inthe macroscopic angle. A second magnetic field, RF field (field B1), maybe applied to M, oscillating the Larmor frequency, and causing M toprecess away from the field B0 direction. During the excitation by radiofrequency, longitudinal magnetization may decrease and transversemagnetization may appear. Merely by way of example, if an excitation RFpulse with a 90° flip angle is applied, when the RF transmitter isturned off, there is no longitudinal magnetization any more, and onlytransverse magnetization exists. The transverse magnetization may inducea current signal in the RF receiving coils, and the induced current maybe referred to as an MR signal.

After the RF excitation with a 90° excitation RF pulse is turned off,the transverse magnetization may decay. Note that the excitation RFpulse may have a flip angle other than 90°, e.g., any magnitude rangingfrom 0° to 180°. An excitation RF pulse with a flip angle of 90° ismentioned elsewhere in the present disclosure for illustration purposes,and is not intended to limit the scope of the present disclosure. Insome embodiments, the decay may be approximated by an exponential curve,which is illustrated by the T2-relaxation shown in FIG. 4. TheT2-relaxation (spin-spin relaxation) may be due to spins getting out ofphase (or referred to as “dephase”). Since at least some nucleus spinsmay move together, their magnetic fields may interact with each other,and may cause a change in their precession rate. As these interactionsare random and temporary, they may cause an accumulative loss in phaseand lead to transverse magnetization decay. T2 may be defined as thetime needed for the transverse magnetization to fall to 1/e or about 37%of its maximum value in FIG. 4. The T1-relaxation (spin-latticerelaxation) may result from energy exchange between the nucleus spinsand their surrounding lattices, during which the spins go from a highenergy state toward a thermal equilibrium state. As illustrated in FIG.4, T1 may be defined as the time needed for the longitudinalmagnetization to reach (1-1/e) or about 63% of its maximum value. At thesame time, the longitudinal magnetization may recover followingapproximately an exponential form, which may be referred to asT1-relaxation shown in FIG. 4. It should be noted that for differentsubjects (e.g., tissues), their T1 and/or T2 are usually different fromeach other even when they are subject to the same magnet field. Forexample, with a 1.5 T field strength, T1 of white matter, gray matter,and cerebrospinal fluid (CSF) of the brain are approximately 350˜500,400˜600, 3000˜4000 milliseconds, respectively. It should also be notedthat T1 and T2 may be different from each other for a same tissue of asame subject under a same magnet field. For example, with a 1.5 T fieldstrength, T1 of white matter of the brain may be about 350˜500milliseconds, while T2 of white matter of the brain may be about 90˜100milliseconds, which is shorter than the T1. T2-relaxation may existregardless of whether there is a T1-ralaxation. Some processes mayresult in or affect T2-relaxation but without affecting T1-relaxation.T1-relaxation may be slower than T2-relaxation. The T1 value may belonger than or equal to the corresponding T2 value.

The T2-relaxation may be exploited to generate an MR signal to image asubject. A spin echo based method may be used in an MRI system toprolong T2 relaxation time. The term “spin echo” or “spin echo sequence”may generally refer to an echo or several echoes formed after theapplication of, for example, two RF pulses including an excitation RFpulse and a refocusing RF pulse. The spin echo and/or spin echo sequenceincludes single spin echo, multi-echo spin echo sequence, fast spin echo(FSE, or turbo spin echo (TSE)) sequence, etc.

Merely by way of example of a single spin echo, a 90°-excitation RFpulse may tip the spins into the transverse plane. Then a refocusing RFpulse may turn the spins. The refocusing RF pulse may be used to reduceor prevent the dephasing caused by the non-uniformity of the main magnetfield and preserve the real T2-ralaxation. The single spin echo maygenerate one MR signal (e.g., an echo) during the course of theT2-relaxation. The MR signal may be used to generate an image.

As another example, a multi-spin echo sequence may be explained asfollows: after the first echo is obtained, there may be an intervaluntil the next repetition time (TR). By applying another refocusing RFpulse, another echo may occur and be detected, with the same phaseencoding, to build another image. The other image may be of a differentcontrast, and may be used in characterizing certain feature of an ROI,e.g., one or more lesions in the ROI. The multi-spin echo may buildseveral images of several slices of the same positioning of a subjectwithout increasing the overall acquisition time by using an interleavedscanning manner. The term “repetition time” or “TR” may refer to thetime between the applications of two consecutive excitation RF pulses.The term “slice” here may refer to a planar region being excited by aspatial excitation RF pulse.

FIG. 5 is a graph of an exemplary flip angle schedule applicable in fastspin echo based magnetic resonance imaging according to some embodimentsto the present disclosure. According to the fast spin echo technique,after the first echo is detected, within the time interval between theexcitation RF pulse and the last refocusing RF pulse within a same TR,an echo train is detected, which may include one or more echoes, to fillthe k-space lines in the same slice. Because multiple echoes may bedetected within one TR, a number of repetitions needed to fill thek-space may be reduced, the k-space may be filled faster, and the sliceacquisition time may be reduced. This may be achieved by applyingseveral 180° refocusing RF pulses to obtain an echo train. As describedelsewhere in the present disclosure, the flip angle of a refocusing RFpulse may be of a value other than 180°. After each echo, thephase-encoding may be cancelled and a different phase-encoding may beapplied to the following echo. The number of echoes received within asame TR is called the echo train length (ETL). The echo train length(ETL) may be one, two, three, or more than three. In some embodiments,using refocusing RF pulses with flip angles of 180°, the specificabsorption rate (SAR) may increase significantly and the T2-relaxationmay be remarkable during imaging. In this condition, the ETL may need tobe set short, e.g., no more than 30. In some embodiments, usingrefocusing RF pulses with variable flip angles, the ETL may be longer,e.g., more than 30. The ETL may be several hundred, or higher.

As also illustrated in FIG. 5, in some embodiments of multi-spin echo,echo time (TE) is referred to as the time between the middle of anexcitation RF pulse and the middle of the spin echo production. As usedherein, “middle” may refer to when the intensity of an echocorresponding to a pulse, e.g., an excitation RF pulse, a refocusing RFpulse, arrives at a maximum value as illustrated in FIG. 5. For amulti-echo train, echo times may be denoted as TE1, TE2, etc. In someembodiments of fast spin echo, as the echoes corresponding to thecentral k-space lines are the ones that may determine image contrast,the time between the middle of an exciting RF pulse and the middle ofthe echoes corresponding to the central k-space is called effective echotime (effective TE, or TE_(eff)).

In some embodiments according to the present disclosure, the differencein the characteristic, e.g., T1 value, T2 value, and/or proton density(or spin density), among different subjects (e.g., different issues) mayprovide a basis to show an anatomic structure and/or pathologicalchanges in magnetic resonance imaging. Several weighting imaging typesmay be used to emphasize above characteristics and build specificimages. Exemplary imaging type may include T1 weighted imaging (T1WI),T2 weighted imaging (T2WI), proton density weighted imaging (PDWI), orthe like, or any combination thereof. For example, in T1 weightedimaging, the differences in longitudinal relaxation of different subjectare emphasized, but the effect of other characteristics, e.g., thedifferences in transverse relaxation, may be de-emphasized or depressed.T1 weighted imaging may have short TE and TR times. As another example,T2 weighted imaging exploits the transverse relaxation of the subjects,and de-emphasized or depressed other characteristics, e.g., longitudinalrelaxation. T2 weighted imaging may need long TE and TR times. As stillanother example, proton density weighted imaging may reflect the proton(in the form of water or macromolecules, etc.) concentration ofdifferent subjects.

It should be noted that the above description of the spin echo sequenceis merely provided for the purposes of illustration, and not intended tolimit the scope of the present disclosure. For persons having ordinaryskills in the art, multiple variations or modifications may be madeunder the teachings of the present disclosure. For example, the flipangle of a refocusing RF pulse may be of a value other than 180°; it maybe any proper value chosen from 0˜180°. As another example, TR or ETLmay be changed or selected according to variations or modificationswithout departing from the scope of the present disclosure.

FIG. 6 is a block diagram of the processing unit 130 according to someembodiments of the present disclosure. The processing unit 130 asillustrated in FIG. 1 and FIG. 6 may process information before, during,or after an imaging procedure. The processing unit 130 may have someother variations, and FIG. 6 is provided for illustration purposes. Theprocessing unit 130 may be implemented as a central processing unit(CPU), an application-specific integrated circuit (ASIC), anapplication-specific instruction-set processor (ASIP), a graphicsprocessing unit (GPU), a physics processing unit (PPU), amicrocontroller unit, a digital signal processor (DSP), a fieldprogrammable gate array (FPGA), an ARM, or the like, or any combinationthereof. As shown in FIG. 6, the processing unit 130 may include acomputing module 601, an image processing module 602, and a storagemodule 603.

The computing module 601 may be used for calculating different kinds ofinformation received from the control unit 120 and/or the display unit140. The information from the control unit 120 may include informationabout the MRI scanner 110, the gradient magnet field generator 111, themain magnet field generator 113, a subject position (e.g., within an MRIsystem), the RF transmit/receive unit 112, or the like, or anycombination thereof. In some embodiments, the information may be asubject position, the main and/or gradient magnet intensity, the radiofrequency phase and/or amplitude, and so on. The information from thedisplay unit 140 may include information from a user and/or otherexternal resource. Exemplary information from a user may includeparameters regarding image contrast and/or ratio, an ROI (e.g., the typeof tissue to be imaged, etc.), slice thickness, an imaging type (e.g.,T1 weighted imaging, T2 weighted imaging, proton density weightedimaging, etc.), T1, T2, a spin echo type (e.g., spin echo, fast spinecho (FSE), fast recovery FSE, single shot FSE, gradient recalled echo,fast imaging with stead-state procession, and so on), a flip anglevalue, acquisition time (TA), echo time (TE), repetition time (TR), echotrain length (ETL), the number of phases, the number of excitations(NEX), inversion time, bandwidth (e.g., RF receiver bandwidth, RFtransmitter bandwidth, etc.), or the like, or any combination thereof.

The image processing module 602 may process the data such as magneticresonance (MR) signals acquired from the ROI and reconstruct them intoan MR image. The image processing module 602 may or may not include animage reconstruction block. The image processing module 602 mayspatially decode MR signals that has been spatially encoded by themagnetic field(s). The intensity or magnitude of the signal, and otherproperties such as a phase number, a relaxation time (T1 or T2),magnetization transfer, or the like, may be ascertained. The imageprocessing module 602 may employ different kinds of imagingreconstruction techniques for the image reconstruction procedure. Theimage reconstruction methods may include parallel imaging, Fourierreconstruction, constrained image reconstruction, compressed sensing, orthe like, or a variation thereof, or any combination thereof. As fordimensions, the Fourier transformation may include 1-dimensional (1D)Fourier transformation, 2-dimensional (2D) Fourier transformation,3-dimensional (3D) Fourier transformation. As for types, the Fouriertransformation may include discrete Fourier transformation, inverseFourier transformation, fast Fourier transformation (FFT), non-uniformfast Fourier transformation (NUFFT), partial Fourier transformation, orthe like, or any combination thereof. Exemplary algorithms of parallelimaging may include simultaneous acquisition of spatial harmonics(SMASH), sensitivity encoding (SENSE), or the like, or any combinationthereof. Exemplary algorithms of partial Fourier transformation mayinclude zero filling, homodyne processing, iterative homodyneprocessing, or the like, or any combination thereof.

In some embodiments of the present disclosure, the received MR signals,for example, echoes generated by applying a plurality of refocusing RFpulses, may be used for k-space sampling. The k-space sampling may beperformed based on encoding gradients applied to the received MRsignals. The k-space sampling may include Cartesian sampling,non-Cartesian sampling, or the like. The encoding gradients may includea slice selection gradient, a phase encoding gradient, a frequencyencoding gradient, or the like, or any combination thereof. Thewaveforms of encoding gradients may be periodic, or aperiodic. As tofast imaging, the k-space may be undersampled and one or moreundersampled k-space data sets may be generated. The imagereconstruction methods described above may be used to reconstruct MRimage based on the undersampled data sets. Merely by way of example, anx-direction encoding gradient and a y-direction encoding gradient may beperformed on the MR signals simultaneously to generate undersampledk-space data sets. As other examples, the x-direction encoding gradientand the y-direction gradient may be performed sequentially regardless ofthe order.

The storage module 603 may store the information that may be used by thecomputing module 601 and/or the image processing module 602. Theinformation may include programs, software, algorithms, data, text,number, images and some other information. These examples are providedhere for illustration purposes, and not intended to limit the scope ofthe present disclosure. Algorithms stored in the storage module 603 mayinclude recursion, a bisection method, an exhaustive search (orbrute-force search), a greedy algorithm, a divide and conquer algorithm,a dynamic programming method, an iterative method, a branch-and-boundalgorithm, a backtracking algorithm, or the like, or any combinationthereof.

It should be noted that the above description of the processing unit ismerely provided for the purposes of illustration, and not intended tolimit the scope of the present disclosure. For persons having ordinaryskills in the art, multiple variations or modifications may be madeunder the teachings of the present disclosure. For example, the assemblyand/or function of processing unit may be varied or changed. In someembodiments, the computing module 601 and the image processing module602 may share one storage module 603. While in some embodiments, thecomputing module 601 and the image processing module 602 may have theirown storage blocks, respectively. However, those variations andmodifications do not depart from the scope of the present disclosure.

FIG. 7 is a flowchart illustrating a process of the processing unit 130according to some embodiments of the present disclosure. Referring toFIG. 7, RF signals may be transmitted in step 701. The RF signals may beused to excite an ROI of a subject to generate MR signals. In someembodiments, the RF signals may include an excitation RF pulse and aplurality of refocusing RF pulses. Following the RF signals, a pluralityof MR signals, for example, a plurality of echoes may be generated. Theechoes may either be spin echoes or gradient echoes.

The MR signals generated in step 701 may be received in step 702. Insome embodiments, one or more echoes may be received in step 702 byapplying encoding gradients on the echoes to spatially encode theechoes. The encoding gradients may include a slice selection gradient, aphase encoding gradient, a frequency encoding gradient, or the like, orany combination thereof. The k-space sampling may be performed based onthe processing of the MR signals.

MR images may be generated based on the MR signals received in step 703.In some embodiments, image reconstruction methods may be applied on theMR signals, for example, echoes, to generated MR images. In someembodiments, the echoes described in step 702 may be undersampled sothat undersampled k-space data sets may be generated and the k-space maybe partially filled. Using an image reconstruction method such ascompressed sensing (CS), parallel imaging, and partial Fouriertransformation, one or more MR images may be generated.

It should be noted that the flowchart described above is merely providedfor the purposes of illustration, and not intended to limit the scope ofthe present disclosure. For persons having ordinary skills in the art,various modifications and variations may be conducted under the teachingof the present disclosure. However, those modifications and variationsmay not depart from the scope of the present disclosure.

FIG. 8 is a block diagram of the image processing module 602 accordingto some embodiments of the present disclosure. Referring to FIG. 8, theimage processing module 602 may include a signal acquisition block 801,a k-space sampling block 802, a Fourier transformation block 803, animage reconstruction block 804, a parameter configuration block 805, anda gradient configuration block 806.

The signal acquisition block 801 may acquire signals, for example, MRsignals. The MR signals may include spin echoes, gradient echoes, or thelike, or any combination thereof.

The k-space sampling block 802 may perform k-space sampling. In someembodiments of the present disclosure, k-space sampling may be performedbased on the MR signals acquired by the signal acquisition block 801.

The Fourier transformation block 803 may preform Fourier transformation.The Fourier transformation may include inverse Fourier transformation,discrete Fourier transformation, fast Fourier transformation, partialFourier transformation, non-uniform fast Fourier transformation, or thelike, or any combination thereof.

The image reconstruction block 804 may generated MR images. In someembodiments of the present disclosure, the MR signals acquired by thesignal acquisition block 801 may be undersampled by the k-space samplingblock 802 to generate undersampled k-space data sets. The undresampledk-space data sets may be used to generate one or more MR images.

The image reconstruction block 804 may receive the undersampled k-spacedata sets and perform image reconstruction based on the undersampledk-space data sets. The image reconstruction may be performed based onone or more image reconstruction methods. Exemplary image reconstructionmethods may include compressed sensing (CS), parallel imaging, partialFourier transformation, or the like, or any combination thereof.

The parameter configuration block 805 may set parameters relating to MRimages. The parameters may include a reduction factor, an imageresolution, image contrast and/or ratio, an ROI, slice thickness, animaging type (e.g., T1 weighted imaging, T2 weighted imaging, protondensity weighted imaging, etc.), T1, T2, a spin echo type (spin echo,fast spin echo (FSE), fast recovery FSE, single shot FSE, gradientrecalled echo, fast imaging with stead-state procession, and so on), aflip angle value, acquisition time (TA), echo time (1E), repetition time(TR), echo train length (ETL), the number of phases, the number ofexcitations (NEX), inversion time, bandwidth (e.g., RF receiverbandwidth, RF transmitter bandwidth, etc.), or the like, or anycombination thereof.

The gradient configuration block 806 may control various kinds ofgradients applied to MR signals. The gradients may include a sliceselection gradient, a phase encoding gradient, a frequency encodinggradient, or the like, or any combination thereof.

It should be noted the description of the image processing module isprovided for the purposed of illustration, and not intended to limit thescope of the present disclosure. For persons having ordinary skills inthe art, various modifications and variations may be conducted under theteaching of the present disclosure. However, those variations andmodifications may not depart from the present disclosure.

FIG. 9 is a flowchart illustrating an image processing according to someembodiments of the present disclosure. MR signals may be acquired instep 901. The digitization of MR signals may be called signalacquisition or signal sampling. MR signals received by receiving coilsmay be radio waves with spatial encoding information, which may belongto analog signals rather than digital signals. In some embodiments,digital signals may propagate more efficiently than analog signals.Digital signals, which may be well-defined and orderly, may be easierfor electronic circuits to distinguish from noise, which may be chaotic.This may be one of many advantages of digital signals in communications.Therefore, an analog signal may be converted into a digital signal.Analog to digital conversion may include sampling, holding,quantization, and encoding. In some embodiments, the device for signalacquisition may be an Analog to Digital Converter (ADC). According tothe principle of operation, the types of ADC may includedirect-conversion ADC, successive approximation ADC, ramp-compare ADC,Wilkinson ADC, integrating ADC, delta-encoded ADC, pipeline ADC,sigma-delta ADC, time-interleaved ADC and time stretch ADC. One or moreof the above ADC may be used in the system and method disclosed herein.

It should be noted that the above description is merely provided for thepurposes of illustration, and not intended to limit the scope of thepresent disclosure. For persons having ordinary skills in the art,multiple variations and modifications may be made under the teaching ofthe present invention. For example, in some embodiments, processes ofsampling and holding may combine to make a sampling/holding process. Insome embodiments, processes of quantization and encoding may beimplemented simultaneously. However, those variations and modificationsdo not depart from the scope of the present disclosure.

The digitized MR signals may be processed in step 902. In someembodiments, data acquired by analog to digital conversion (ADC data) instep 901 are not used for image reconstruction directly due to lack ofsome information (e.g. control information, identification information,etc.). Therefore, some information for image reconstruction may need tobe added into ADC data. These information may include information abouta scan counter, the type of ADC data, the gating data of a physiologicalsignal, or the like, or a combination thereof. The methods to processADC data may include data registration, pre-processing beforereconstruction, or the like, or any combination thereof.

The k-space sampling may be performed based on the processed MR signalsin step 903. The k-space sampling may include filling k-space with aplurality of k-space trajectories. The filling method may vary accordingto certain conditions. For example, the k-space trajectory may be aCartesian trajectory or a non-Cartesian trajectory. The non-Cartesiantrajectory may be radial, spiral, zigzag, propeller, or the like, or anycombination thereof. The k-space may be undersampled, oversampled, orfully sampled. The data may be filled into the k-space in any order,such as from the center outward, from left to right, from the top down.The data generated from step 903 may be called k-space data.

It should be noted that the above description is merely provided for thepurposes of illustration, and not intended to limit the scope of thepresent disclosure. For persons having ordinary skills in the art,multiple variations and modifications may be made under the teaching ofthe present invention. For example, in some embodiments, the k-space maybe oversampled in the center region and undersampled in the peripheralregion. Alternatively, the k-space may be fully sampled in the centerregion and undersampled in a peripheral region. It may vary according tocertain conditions. However, those variations and modifications do notdepart from the scope of the present disclosure.

Inverse Fourier transformation may be performed on the k-space data instep 904. The k-space data may be transformed into frequency-domain datawhich may be a two-dimensional (2-D) or three-dimensional (3-D) dataset. Exemplary image reconstruction methods may include parallelimaging, Fourier reconstruction, constrained image reconstruction,compressed sensing, or the like, or a variation thereof, or anycombination thereof. As for dimensions, the Fourier transformation mayinclude 1-dimensional (1D) Fourier transformation, 2-dimensional (2D)Fourier transformation, 3-dimensional (3D) Fourier transformation. Asfor types, the Fourier transformation may include discrete Fouriertransformation, inverse Fourier transformation, fast Fouriertransformation (FFT), non-uniform fast Fourier transformation (NUFFT),partial Fourier transformation, or the like, or any combination thereof.Exemplary algorithms of partial Fourier transformation may include zerofilling, homodyne processing, iterative homodyne processing, or thelike, or any combination thereof. Exemplary algorithms of parallelimaging may include simultaneous acquisition of spatial harmonics(SMASH), AUTO-SMASH, VD-AUTO-SMASH, sensitivity encoding (SENSE),generalized autocalibrating partially parallel acquisitions (GRAPPA), orthe like, or any combination thereof.

In step 905, the frequency-domain data may be processed to generate MRimages. In some embodiments, the frequency-domain data may be mappedinto optical data for further video processing. For example, in amonochrome display, the frequency-domain data may be mapped into theluminance values of pixels. In a color display, frequency-domain datamay be mapped into the luminance and false-color values of pixels. Theoptical data then may be transformed into signals which drive pixels ona display.

It should be noted that the above description is merely provided for thepurposes of illustration, and not intended to limit the scope of thepresent disclosure. For persons having ordinary skills in the art,multiple variations and modifications may be made under the teaching ofthe present invention. For example, in some embodiments, after step 905,there may be further processing on generated images, such as geometrictransformation and adjustment of image quality. The geometrictransformation may include rotation, cropping, mirroring, zooming, orthe like, or any combination thereof. The adjustment of image qualitymay include false-color processing, sharpening, image intensification,or the like, or any combination thereof. However, those variations andmodifications do not depart from the scope of the present disclosure.

FIG. 10 illustrates a flowchart of the k-space sampling according tosome embodiments of the present disclosure. A slice selection gradientmay be started in step 1001. The slice selection gradient may select aslice of an ROI. In some embodiments of the present disclosure, theslice selection gradient may be turned on as an RF pulse is applied. TheRF pulse may be an excitation RF pulse, a refocusing RF pulse, etc. Theslice selection gradient may be turned off when the application of theRF pulse stops.

A first encoding gradient may be turned on in step 1002, and a secondencoding gradient may be turned on in step 1003. The first encodinggradient and the second encoding gradient may spatially encode MRsignals received by the image processing module as described elsewherein the present disclosure. In some embodiments, the first encodinggradient may be a phase encoding gradient, and the second encodinggradient may be a frequency encoding gradient. As an example, the phaseencoding gradient is applied first and the frequency encoding gradientis applied subsequently. As a result, a Cartesian k-space trajectory maybe generated. The Cartesian k-space trajectories may include a certainnumber of straight lines.

In some embodiments of the present disclosure, the first encodinggradient and the second encoding gradient may be applied simultaneouslyon MR signals. Accordingly, a non-Cartesian k-space trajectory may begenerated. The non-Cartesian k-space trajectory may be radial, spiral,or the like, or a combination thereof. The MR signals may be encodedbased on the first encoding gradient and the second encoding gradient instep 1004.

In step 1005, the k-space sampling may be performed based on the encodedMR signals generated in step 1004. Specifically, exemplary trajectoriesfor the k-space sampling may include a Cartesian trajectory or anon-Cartesian trajectory. The non-Cartesian trajectory may be radial,spiral, zigzag, propeller, or the like, or any combination thereof. Thek-space may be undersampled, oversampled, or fully sampled. In someembodiments, the k-space may be fully sampled in the center region andundersampled in a peripheral region. As used herein, if the k-space issymmetrical with respect to the line of Ky=0, a center region may referto the region in which the absolute value of Ky is small. A peripheralregion may refer to the region in which the absolute value of Ky islarge. The data may be filled into k-space in any order, such as fromthe center outward, from left to right, from the top down, etc.

It should be noted that the flowchart described above is provided forthe purposes of illustration, and not intended to limit the scope of thepresent disclosure. For persons having ordinary skills in the art,various modifications and variations may be conducted under the teachingof the present disclosure. However, those modifications and variationsmay not depart from the present disclosure. For example, step 1002 andstep 1003 may be performed either concurrently or sequentially.

FIG. 11 shows a diagram of a 2D FSE pulse sequence for a singlerepetition time (TR) according to some embodiments of the presentdisclosure. The 2D FSE pulse sequence shown in FIG. 11 is suitable forapplication in the MRI system described elsewhere in the presentdisclosure. The 2D FSE pulse sequence 1100 may include an excitation RFpulse 1101 and a plurality of refocusing RF pulses, for example, therefocusing RF pulse 1102, the refocusing RF pulse 1103, the refocusingRF pulse 1104, and the refocusing RF pulse 1105. The excitation RF pulse1101, along with the slice selection gradient 1111 may be used to tip aportion of the longitudinal magnetization of an ROI into the transverseplane. Following the excitation RF pulse 1101, a plurality of refocusingRF pulses may be transmitted to generate a plurality of echoes. Arefocusing RF pulse may be accompanied with a slice selection gradient.The number of echoes generated may be determined by the number ofrefocusing RF pulses.

As illustrated in FIG. 11, the echo 1141 may be generated by therefocusing RF pulse 1112, the echo 1142 may be generated by therefocusing RF pulse 1113, and the echo 1143 may be generated by therefocusing RF pulse 1114. The x-direction encoding gradient and they-direction encoding gradient may be used to spatially encode theechoes. For example, the x-direction encoding gradient 1121 and they-direction encoding gradient 1131 may be applied to the echo 1141 sothat the echo 1141 is spatially encoded. A spatially encoded echo maycorrespond to a k-space trajectory. The spatially encoded echo may beused to perform k-space sampling and construct MR images subsequently.

In some embodiments of the present disclosure, an x-direction encodinggradient and a y-direction encoding gradient may be applied to an echosimultaneously to generate a k-space trajectory. It should be noted thatthe k-space trajectory may include either a straight line or a curvewhich correspond to Cartesian sampling or non-Cartesian samplingrespectively in MRI. The non-Cartesian sampling may include radialsampling and spiral sampling.

It should be noted that the echoes generated by the refocusing RF pulsesmay be echoes whose flip angles are 180°. In some embodiments of thepresent disclosure, a flip angle schedule in which flip angles arevaried may be applied to the refocusing RF pulses. For example, becauseof T2-relaxation, the intensity of each echo in one echo train may bedifferent. As used herein, the T2-relaxation may refer to theprogressive dephasing of spinning dipoles following the 90° pulse asseen in a spin echo sequence due to tissue-particular characteristics.So if the echo train length (ETL) is 4, in order to decrease thedifference among the echoes, the flip angles of the refocusing RF pulsesmay be set as the values which are less than or equal to 180°, such as140°, 155°, 165°, 180° in turn.

In 2D FSE, a plurality of echoes may be acquired in a repetition time(TR). In some embodiments, a non-Cartesian coordinate system may beemployed in the present disclosure. The slice selection gradient may beapplied in the z-direction to select a slice of an ROI to be imaged. Thex-direction encoding gradient and the y-direction encoding gradient maybe used to spatially encode MR signals, for example echoes, toaccomplish the k-space sampling. The k-space sampling may be performedby filling k-space with a certain number of k-space trajectories.Exemplary k-space sampling methods may include Cartesian sampling andnon-Cartesian sampling. As for Cartesian sampling, k-space may be filledwith Cartesian trajectories, for example, straight lines. As fornon-Cartesian sampling, k-space may be filled with non-Cartesiantrajectories other than Cartesian trajectories, for example, radial orspiral trajectories. In the process of acquiring an echo, the encodinggradients may be employed in the x and y axes to fill a two-dimensionk-space in which Kx and Ky may be used as the coordinates.

During the process of acquiring an echo, the amplitude of thex-direction encoding gradient may be kept either positive or negative.The amplitude of the x-direction encoding gradient when acquiring highspatial frequency information (e.g. edges, details, sharp transitions)of the k-space may be larger than the amplitude of the x-directionencoding gradient when acquiring low spatial frequency information (e.g.contrast, general shapes) of the k-space. From the acquisition of thehigh spatial frequency information to that of the low spatial frequencyinformation, the amplitude of the x-direction encoding gradient may varygradually. In some embodiments, the transition from high amplitude tolow amplitude of the x-direction encoding gradient may be sharp.

FIGS. 12A-12F show exemplary waveforms of x-direction encoding gradientsaccording to some embodiments of the present disclosure. For furtherillustrating the present disclosure, several examples are given below,but the examples do not limit the scope of the present disclosure.

FIG. 12A illustrates an exemplary waveform of an x-direction encodinggradient according to some embodiments of the present disclosure. Asillustrated in FIG. 12A, the waveform of the x-direction encodinggradient may include three steady phases (phase A, phase B, and phase C)and two phases of transition. Phase A and phase C may be used to acquirehigh spatial frequency information in k-space. Phase B may be used toacquire low spatial frequency information in k-space. The letter t maydenote the time duration for the acquisition of an echo. The amplitudein phased A, phase B, and phase C may be kept unchanged. The two phasesof transition may be linear function. In some embodiments, the twophases of transition may be nonlinear function.

FIG. 12B illustrates an exemplary waveform of an x-direction encodinggradient according to some embodiments of the present disclosure. Asillustrated in FIG. 12B, the x-direction encoding gradient may be partof a function that has a smooth variation. The function may be aGaussian function, a harmonic function, or the like, or any combinationthereof. As illustrated in FIG. 12B, the edges of the x-directionencoding gradient (A and C) with larger amplitude may be used to acquirehigh spatial frequency information. The center region of the x-directionencoding gradient (B) with lower amplitude may be used to acquire lowspatial frequency information. The letter t may denote the time durationfor the acquisition of an echo.

In some embodiments, instead of being symmetric, the waveform of thex-direction encoding gradient may be asymmetric as shown in FIG. 12E andFIG. 12F. The portion as indicated by B in FIG. 12E that has a loweramplitude may be used to acquire low spatial frequency information(e.g., close to the center region of k-space). The portions A and C inFIG. 12E that have higher amplitudes may be used to acquire high spatialfrequency information (e.g., a peripheral region of k-space). Likewise,the portion B in FIG. 12F that has lower amplitude may be used toacquire low spatial frequency information (e.g., close to the centerregion of k-space). The portions A and C in FIG. 12F that have higheramplitude may be used to acquire high spatial frequency information(e.g., a peripheral region of k-space).

In some embodiments, when k-space is undersampled, the x-directionencoding gradient as shown in FIG. 12E and FIG. 12F may be used toperform k-space sampling.

It should be noted that the portion that has lower amplitude of anx-direction encoding gradient may be located in anywhere in the waveformof the x-direction encoding gradient, for example, in the center of thewaveform, in the left of the waveform, or in the right of the waveform.

It should be noted that the above description of the waveforms of thex-direction encoding gradient are merely provided for the purposes ofillustration, and not intended to limit the scope of the presentdisclosure. For persons having ordinary skills in the art, multiplevariations and modifications may be made under the teachings of thepresent disclosure. For example, in some embodiments, the time durationfor acquiring high spatial frequency information and low spatialfrequency information of k-space for an x-direction encoding gradientmay be equal or unequal. As other examples, the time of acquiring highspatial frequency information may be longer or shorter than that ofacquiring low spatial frequency information. However, those variationsand modifications do not depart from the scope of the presentdisclosure.

In some embodiments, there may be a dephasing gradient before theapplication of an x-direction encoding gradient and/or the applicationof a rephrasing gradient after an x-direction encoding gradient. In theprocess of acquiring an echo, the dephasing gradient may be used todetermine the starting point of a k-space trajectory of the echo. Therephasing gradient may adjust the state of the echo. In someembodiments, the rephasing gradient may eliminate the effects caused bythe dephasing gradient.

For further understanding the present disclosure, several examples aregiven below, but the examples do not limit the scope of the presentdisclosure. FIG. 12C illustrates exemplary waveforms of an x-directiondephasing gradient, an x-direction rephrasing gradient, and anx-direction encoding gradient according to some embodiments of thepresent disclosure. As shown in FIG. 12C, the waveform of the firstencoding gradient that includes three steady phases and two phases oftransition may be used to acquire an echo. The letter A may denote adephasing gradient, and the letter B may denote a rephasing gradient.The letter t may denote the time duration for the acquisition of anecho. The amplitudes of the dephasing gradient and the rephasinggradient may have a same direction (positive or negative).Alternatively, the amplitudes of the dephasing gradient and therephasing gradient may have opposite directions (positive or negative).Exemplary waveforms of the dephasing gradient and the rephasing gradientmay be a trapezoidal wave, a square wave, a triangular wave, or thelike, or a combination thereof.

It should be noted that the above description of the dephasing gradientand the rephasing gradient is merely provided for the purposes ofillustration, and not intended to limit the scope of the presentdisclosure. For persons having ordinary skills in the art, multiplevariations and modifications may be made under the teachings of thepresent disclosure. For example, the dephasing gradients in x-directionof two successive echoes may be slightly different in amplitude. Forexample, in some embodiments, when different echoes are acquired, thedephasing gradients may be adjusted to make the starting points of thek-space trajectories different. However, those variations andmodifications do not depart from the scope of the present disclosure.

The x-direction encoding gradients, which are employed to acquiredifferent echoes, may have the same pattern or different patterns. Insome embodiments, the patterns of the x-direction encoding gradients maybe made different by adjusting the amplitudes, the waveforms, the timeduration for the acquisition of an echo and/or the time duration of eachphrase.

For further understanding the present disclosure, several examples aregiven below, but the examples do not limit the scope of the presentdisclosure. FIG. 12D illustrates waveforms of two successive x-directionencoding gradients that may be used to acquire two successive echoesaccording to some embodiments of the present disclosure. As illustratedin FIG. 12D, the solid line may denote the x-direction encoding gradientthat may be used to acquire echo n, and the dash line may denote thex-direction encoding gradient that may be used to acquire echo m. Theletter t may denote the time duration for the acquisition of an echo.Both the two x-direction encoding gradients have three steady phases andtwo phases of transition. However, they may have different amplitudesand different time intervals for each phase. As shown in FIG. 12D, theamplitude of the first steady phase of the x-direction encoding gradientfor echo m may be larger than that of echo n. The time interval for thephase of transition of the x-direction encoding gradient for echo m maybe longer than that of echo n. The time for acquiring the high spatialfrequency information of the x-direction encoding gradient for echo mmay be shorter than that of echo n. The time for acquiring the lowspatial frequency information of the x-direction encoding gradient forecho m may be longer than that of echo n.

It should be noted that the above description of the waveforms of thex-direction encoding gradients is merely provided for the purposes ofillustration, and not intended to limit the scope of the presentdisclosure. For persons having ordinary skills in the art, multiplevariations and modifications may be made under the teachings of thepresent disclosure. For example, the waveforms of the x-directionencoding gradients for acquiring different echoes may be different. Insome embodiments, the x-direction encoding gradient for echo m mayinclude three steady phases and two phases of transition. While thex-direction encoding gradient for echo n may be part of a smoothfunction. However, those variations and modifications do not depart fromthe scope of the present disclosure.

In some embodiments of the present disclosure, when acquiring an echo, ay-direction encoding gradient may be applied simultaneously with anx-direction encoding gradient. In the process of echo acquisition, theamplitude of the y-direction encoding gradient may oscillate aroundzero, and the k-space trajectory may oscillate around a baseline aswell. The baseline may correspond to a value of Ky in the k-space. Theoscillation of the amplitude of the y-direction encoding gradient may beperiodic or aperiodic.

For further illustrating the present disclosure, several examples aregiven below, but the examples do not limit the scope of the presentdisclosure. FIG. 13A illustrates an exemplary waveform of a y-directionencoding gradient according to some embodiments of the presentdisclosure. As illustrated in FIG. 13A, the y-direction encodinggradient may be a trapezoidal wave. The amplitude of the y-directionencoding gradient oscillates periodically. The letter t may denote thetime duration for the acquisition of an echo. FIG. 13B illustrates anexemplary waveform of a y-direction encoding gradient according to someembodiments of the present disclosure. As illustrated in in FIG. 13B,the y-direction encoding gradient may be a triangular wave. Theamplitude of the y-direction encoding gradient oscillates periodically.The letter t may be the duration of the acquisition of one echo.

It should be noted that the above description of the waveforms of they-direction encoding gradients are merely provided for the purposes ofillustration, and not intended to limit the scope of the presentdisclosure. For persons having ordinary skills in the art, multiplevariations and modifications may be made under the teachings of thepresent disclosure. For example, in some embodiments, the waveform ofthe y-direction encoding gradient may be a sawtooth wave, a square wave,a sinusoidal wave, or the like, or any combination thereof. In someembodiments, within one period, the waveform of the y-direction encodinggradient may be a mixture of any waves mentioned above. In someembodiments, within one period, there may be one kind of waveform butwith different amplitudes. In some embodiments, the amount of the periodin the duration of the acquisition of one echo may be one, two, three,or more. However, those variations and modifications do not depart fromthe scope of the present disclosure.

FIG. 13C illustrates an exemplary waveform of a y-direction encodinggradient according to some embodiments of the present disclosure. Asshown in FIG. 13C, the y-direction encoding gradient is a mixture of atrapezoidal wave, a triangular wave, and a sinusoidal wave in the timeduration for the acquisition of an echo. The y-direction encodinggradient may oscillate in an aperiodic way. The letter t may denote thetime duration for the acquisition of an echo.

It should be noted that the above description of the y-directionencoding gradient is merely provided for the purposes of illustration,and not intended to limit the scope of the present disclosure. Forpersons having ordinary skills in the art, multiple variations andmodifications may be made under the teachings of the present disclosure.For example, the y-direction encoding gradient may oscillate in a randomway as what is described in FIG. 13C. In addition, the y-directionencoding gradient may oscillate in an aperiodic but regular way. Forinstance, in some embodiments, the y-direction encoding gradient mayoscillate in a way where the amplitude of the y-direction encodinggradient may vary (increase or decrease) gradually. However, thosevariations and modifications do not depart from the scope of the presentdisclosure.

The y-direction encoding gradients for acquiring different echoes, forexample, two successive echoes, may be the same or different. In someembodiments, when different echoes are acquired, the patterns of they-direction encoding gradients may be made different by adjusting theamplitudes, the waveforms, the time duration for the acquisition of anecho and/or the oscillating method of the y-direction encodinggradients.

For further understanding the present disclosure, several examples aregiven below, but the examples do not limit the scope of the presentdisclosure. FIG. 13D illustrates waveforms of two exemplary y-directionencoding gradients for acquiring different echoes according to someembodiments of the present disclosure. As shown in FIG. 13D, the solidline may denote the waveform of the y-direction encoding gradient for afirst echo. The dash line may denote the waveform of the y-directionencoding gradient for a second echo other than the first echo. Theletter t may denote the time duration for the acquisition of an echo.The y-direction encoding gradient for acquiring the first echo may be amixture of, for example, a trapezoidal wave, a sawtooth wave, and asinusoidal wave within the time duration for the acquisition of an echo.The y-direction encoding gradient for the first echo may oscillate in anaperiodic way. The y-direction encoding gradient for the second echo maybe a mixed wave as well, which may include the combination of atrapezoidal wave, a sawtooth wave, and a sinusoidal wave within the timeduration for the acquisition of an echo. However, the two y-directionencoding gradients may be different in terms of amplitude and/or a timeinterval of a phase. The y-direction encoding gradient for the secondecho may oscillate in an aperiodic way.

It should be noted that the above description of the waveforms of they-direction encoding gradients are merely provided for the purposes ofillustration, and not intended to limit the scope of the presentdisclosure. For persons having ordinary skills in the art, multiplevariations and modifications may be made under the teachings of thepresent disclosure. For example, the y-direction encoding gradients usedto acquire different echoes may be different in the amplitude, thewaveforms, the amount of the periods in the time duration for theacquisition of an echo, the oscillating method (periodic or aperiodic),or the like, or any combination thereof. However, those variations andmodifications do not depart from the scope of the present disclosure.

In some embodiments, there may be a dephasing gradient before theapplication of a y-direction encoding gradient. In the process ofacquiring an echo, the value of Ky of the baseline may be determined bythe dephasing gradient. For different echoes, the values of Ky of thebaselines may be the same or different.

For further illustrating the present disclosure, several examples aregiven below, but the examples do not limit the scope of the presentdisclosure. FIG. 13E illustrates an exemplary y-direction encodinggradient according to some embodiments of the present disclosure. Asshown in FIG. 13E, an aperiodic waveform including a combination of atriangular wave, a trapezoidal wave, and a sinusoidal wave may be usedto acquire an echo. The letter A may denote the dephasing gradient. Theletter t may denote the time duration for the acquisition of an echo.

In some embodiments, there may be a rephasing gradient after theapplication of a y-direction encoding gradient. The rephasing gradientmay adjust the state of the signal. In some embodiments, the rephasinggradient may eliminate the effects caused by a dephasing gradient.

For further illustrating the present disclosure, several examples aregiven below, but the examples do not limit the scope of the presentdisclosure. FIG. 13F illustrates an exemplary y-direction encodinggradient according to some embodiments of the present disclosure. Asshown in FIG. 13F, the y-direction encoding gradient may be an aperiodicwaveform including a triangular wave, a trapezoidal wave, and asinusoidal wave. The letter A may denote the dephasing gradient, and theletter B may denote the rephasing gradient. The letter t may denote thetime duration for the acquisition of an echo.

It should be noted that the above description about distribution of thedensity of the baseline is merely provided for the purposes ofillustration, and not intended to limit the scope of the presentdisclosure. For persons having ordinary skills in the art, multiplevariations and modifications may be made under the teachings of thepresent disclosure. For example, in some embodiments, the amplitudes ofthe dephasing gradient and the rephasing gradient may be same indirection (positive or negative). Alternatively, the amplitudes of thedephasing gradient and the rephasing gradient may be opposite indirection. The waveforms of the dephasing gradient and the rephasinggradient may be trapezoidal wave, square wave, triangular wave, or thelike. However, those variations and modifications do not depart from thescope of the present disclosure.

The distribution of the density of the baselines in the k-space may beset by employing different dephasing gradients when different echoes areacquired.

FIG. 14 illustrates an exemplary diagram of the distribution ofbaselines of the k-space trajectory according to some embodiments of thepresent disclosure. As shown in FIG. 14, the dots horizontally arrangedmay denote the values of Ky corresponding to baselines. The letter A andthe letter C may denote the peripheral regions of the k-spacecorresponding to the high frequency area, and the letter B may denotethe center regions of k-space corresponding to low frequency area. InFIG. 14, in the low frequency area of the k-space, the density of thebaselines may be higher than that in the high frequency area.

It should be noted that the above description about distribution of thedensity of the baseline is merely provided for the purposes ofillustration, and not intended to limit the scope of the presentdisclosure. For persons having ordinary skills in the art, multiplevariations and modifications may be made under the teachings of thepresent disclosure. For example, in some embodiments, the density of thebaseline may be uniform. In some embodiments, the density of thebaseline may vary in another ways. For instance, the density of thebaselines may vary (increase or decrease) gradually form one side to theother side. In some embodiments, the density of the baselines may be lowin the center region of the k-space and high in the peripheral region ofthe k-space. In some embodiments, the density of the baselines maydistribute in an alternating way. In some embodiments, the density ofthe baseline may also distribute in a random way. However, thosevariations and modifications do not depart from the scope of the presentdisclosure.

FIG. 15 is an exemplary diagram of the k-space sampling according tosome embodiments of the disclosure. The k-space may be sampled by eightk-space trajectories which are non-Cartesian sampling. It should benoted that the amount of echoes used to perform the k-space sampling andthe distribution of baselines in the k-space may be preset. Thedistribution of baselines may be adjusted based on the value of Ky thatmay be controlled by a dephasing gradient that is described elsewhere inthe present disclosure.

In FIG. 15, the vertically distributed black spots may indicate valuesof Ky corresponding to baselines as indicated by dotted straight linesvertically arranged in k-space. A solid line may indicate a k-spacetrajectory of an echo. A k-space trajectory may oscillate based on itsbaseline. In the peripheral region of k-space corresponding to the highfrequency area of k-space, the density of baselines may be lower thanthat in the center region of k-space corresponding to the low frequencyarea. The amplitude of the k-space trajectories may correlate to thedensity of the baselines in k-space. As shown in FIG. 15, in the areawhere the density of the baselines is low, k-space trajectories mayfluctuate sharply. In the area where the density of the baselines ishigh, k-space trajectories may fluctuate gradually. As shown in FIG. 15,the waveforms of the k-space trajectories may be different, but they alloscillate based on the baselines.

EXAMPLES

For further understanding the present disclosure, several examples aregiven below, but the examples do not limit the scope of the presentdisclosure.

FIG. 16 illustrates an exemplary diagram of k-space sampling for waterphantom according to some embodiments of the present disclosure. Asillustrated in FIG. 16, a 2D FSE sequence was used to obtain an MRimage. The size of k-space was 408×408. The reduction factors in theKx-direction and the Ky-direction were both 3.2. The total reductionfactor was 10. One hundred twenty-eight echoes were generated and usedfor the k-space sampling. The ADC acquired 128 data points from an echo.

It should be noted that the above description is merely provided for thepurposes of illustration, and not intended to limit the scope of thepresent disclosure. For persons having ordinary skills in the art,multiple variations and modifications may be made under the teachings ofthe present disclosure. For example, the original size of k-space, thereduction factor, the amount of the acquired echoes and the amount ofdata points acquired from an echo may be changed or selected accordingto variations or modifications without departing from the scope of thepresent disclosure.

FIG. 17A illustrates the original image of the resolution water phantom.FIG. 17B illustrates the image of the resolution water phantomreconstructed by non-uniform fast Fourier transformation (NUFFT). FIG.17C illustrates the image of the resolution water phantom reconstructedby compressed sensing.

As shown in FIG. 17B, compared to the original image of the Shepp Loganwater phantom, when the k-space data obtained according to theundersampling pattern in FIG. 16 were reconstructed by nonuniform fastFourier transformation (NUFFT) and zero filling, the reconstructed imageis unclear, and some spots in the original image may hardly be seen inthe reconstructed image.

As shown in FIG. 17C, when the k-space data obtained according to theundersampling pattern in FIG. 16 were reconstructed by compress sensing,the details of the original image may be restored in the reconstructedimage.

FIG. 18A illustrates the original image of the Shepp Logan waterphantom. FIG. 18B illustrates the image of the Shepp Logan water phantomreconstructed by nonuniform fast Fourier transformation (NUFFT) and zerofilling. FIG. 18C illustrates the image of the Shepp Logan water phantomreconstructed based on compressed sensing.

As shown in FIG. 18B, compared to the original image of the Shepp Loganwater phantom, if the k-space data obtained according to theundersampling pattern in FIG. 16 were reconstructed by nonuniform fastFourier transformation (NUFFT) and using zero to replace the data whichwas not sampled, the reconstructed image is unclear, and some spots inthe original image may hardly be seen in the reconstructed image.

As shown in FIG. 18C, when the k-space data obtained according to theundersampling pattern in FIG. 16 were reconstructed by compress sensing,the details of the original image may be restored in the reconstructedimage.

FIG. 19 illustrates an exemplary diagram of the k-space sampling for aknee according to some embodiments of the present disclosure. Referringto FIG. 19, a 2D FSE sequence was used to obtain MR images. The originalsize of k-space was 384×384. The reduction factors in Kx andKy-direction were both 2. The total reduction factor was 4. One hundredninety-two echoes were generated and used for k-space sampling. The ADCacquired 192 data points from an echo.

It should be noted that the above description is merely provided for thepurposes of illustration, and not intended to limit the scope of thepresent disclosure. For persons having ordinary skills in the art,multiple variations and modifications may be made under the teachings ofthe present disclosure. For example, the original size of k-space, thereduction factor, the amount of the acquired echoes and the amount ofdata points acquired from an echo may be changed or selected accordingto variations or modifications without departing from the scope of thepresent disclosure.

FIG. 20A illustrates the original image of the knee. FIG. 20Billustrates the image of the knee reconstructed by nonuniform fastFourier transformation (NUFFT). FIG. 20C illustrates the image of theknee reconstructed by compressed sensing.

As shown in FIG. 20B, compared to the original image of the knee, if thek-space data obtained according to the undersampling pattern in FIG. 19were reconstructed by nonuniform fast Fourier transformation (NUFFT) andzero filling, the reconstructed image is unclear.

As shown in FIG. 20C, when the k-space data obtained according to theundersampling pattern in FIG. 19 were reconstructed by compress sensing,the details of the original image may be restored in the reconstructedimage.

It should be noted that the above description is merely provided for thepurposes of illustration, and not intended to limit the scope of thepresent disclosure. For persons having ordinary skills in the art,multiple variations and modifications may be made under the teachings ofthe present disclosure. For example, the non-Cartesian samplingdisclosed in this disclosure may combine with techniques includingparallel imaging, compressed sensing, partial Fourier transformation, orthe like, or any combination thereof. However, those variations andmodifications do not depart from the scope of the present disclosure.

Having thus described the basic concepts, it may be rather apparent tothose skilled in the art after reading this detailed disclosure that theforegoing detailed disclosure is intended to be presented by way ofexample only and is not limiting. Various alterations, improvements, andmodifications may occur and are intended to those skilled in the art,though not expressly stated herein. These alterations, improvements, andmodifications are intended to be suggested by this disclosure, and arewithin the spirit and scope of the exemplary embodiments of thisdisclosure.

Moreover, certain terminology has been used to describe embodiments ofthe present disclosure. For example, the terms “one embodiment,” “anembodiment,” and/or “some embodiments” mean that a particular feature,structure or characteristic described in connection with the embodimentis included in at least one embodiment of the present disclosure.Therefore, it is emphasized and should be appreciated that two or morereferences to “an embodiment” or “one embodiment” or “an alternativeembodiment” in various portions of this specification are notnecessarily all referring to the same embodiment. Furthermore, theparticular features, structures or characteristics may be combined assuitable in one or more embodiments of the present disclosure.

Further, it will be appreciated by one skilled in the art, aspects ofthe present disclosure may be illustrated and described herein in any ofa number of patentable classes or context including any new and usefulprocess, machine, manufacture, or composition of matter, or any new anduseful improvement thereof. Accordingly, aspects of the presentdisclosure may be implemented entirely hardware, entirely software(including firmware, resident software, micro-code, etc.) or combiningsoftware and hardware implementation that may all generally be referredto herein as a “block,” “module,” “engine,” “unit,” “component,” or“system.” Furthermore, aspects of the present disclosure may take theform of a computer program product embodied in one or more computerreadable media having computer readable program code embodied thereon.

A computer readable signal medium may include a propagated data signalwith computer readable program code embodied therein, for example, inbaseband or as part of a carrier wave. Such a propagated signal may takeany of a variety of forms, including electro-magnetic, optical, or thelike, or any suitable combination thereof. A computer readable signalmedium may be any computer readable medium that is not a computerreadable storage medium and that may communicate, propagate, ortransport a program for use by or in connection with an instructionexecution system, apparatus, or device. Program code embodied on acomputer readable signal medium may be transmitted using any appropriatemedium, including wireless, wireline, optical fiber cable, RF, or thelike, or any suitable combination of the foregoing.

Computer program code for carrying out operations for aspects of thepresent disclosure may be written in any combination of one or moreprogramming languages, including an object oriented programming languagesuch as Java, Scala, Smalltalk, Eiffel, JADE, Emerald, C++, C#, VB. NET,Python or the like, conventional procedural programming languages, suchas the “C” programming language, Visual Basic, Fortran 2003, Perl, COBOL2002, PHP, ABAP, dynamic programming languages such as Python, Ruby andGroovy, or other programming languages. The program code may executeentirely on the user's computer, partly on the user's computer, as astand-alone software package, partly on the user's computer and partlyon a remote computer or entirely on the remote computer or server. Inthe latter scenario, the remote computer may be connected to the user'scomputer through any type of network, including a local area network(LAN) or a wide area network (WAN), or the connection may be made to anexternal computer (for example, through the Internet using an InternetService Provider) or in a cloud computing environment or offered as aservice such as a Software as a Service (SaaS).

Furthermore, the recited order of processing elements or sequences, orthe use of numbers, letters, or other designations therefore, is notintended to limit the claimed processes and methods to any order exceptas may be specified in the claims. Although the above disclosurediscusses through various examples what is currently considered to be avariety of useful embodiments of the disclosure, it is to be understoodthat such detail is solely for that purpose, and that the appendedclaims are not limited to the disclosed embodiments, but, on thecontrary, are intended to cover modifications and equivalentarrangements that are within the spirit and scope of the disclosedembodiments. For example, although the implementation of variouscomponents described above may be embodied in a hardware device, it mayalso be implemented as a software only solution—e.g., an installation onan existing server or mobile device.

Similarly, it should be appreciated that in the foregoing description ofembodiments of the present disclosure, various features are sometimesgrouped together in a single embodiment, figure, or description thereoffor the purpose of streamlining the disclosure aiding in theunderstanding of one or more of the various inventive embodiments. Thismethod of disclosure, however, is not to be interpreted as reflecting anintention that the claimed subject matter requires more features thanare expressly recited in each claim. Rather, inventive embodiments liein less than all features of a single foregoing disclosed embodiment.

1. A method for generating a magnetic resonance (MR) image, the methodcomprising: generating a main magnetic field through an region ofinterest (ROI); applying a slice selection gradient to a slice of theROI; applying a plurality of RF pulses to the slice of the ROI togenerate a plurality of echoes; applying a first encoding gradient in afirst direction and simultaneously applying a second encoding gradientin a second direction on each echo, wherein the amplitude of the firstencoding gradient for acquiring the center region of a k-space is lowerthan the amplitude of the first encoding gradient for acquiring theperipheral region of the k-space; generating, based on the firstencoding gradient in the first direction and the second encodinggradient in the second direction, a plurality of undersampled k-spacedata sets; and generating an MR image by applying at least one imagereconstruction method to the undersampled k-space data sets.
 2. Themethod of claim 1, the RF pulses comprising fast spin echo (FSE).
 3. Themethod of claim 1, the waveform of the first encoding gradient in thefirst direction comprising three steady phases and two phases oftransition.
 4. The method of claim 1, the waveform of the first encodinggradient in the first direction comprising part of a function having asmooth variation.
 5. (canceled)
 6. The method of claim 1, the secondencoding gradient in the second direction comprising an oscillatingwaveform.
 7. (canceled)
 8. The method of clam 1, the applying a firstencoding gradient in the first direction comprising: applying at leasttwo different encoding gradients for two different echoes, respectively.9. The method of claim 1, the first encoding gradient in the firstdirection further comprising at least one of a dephasing gradient and arephasing gradient.
 10. The method of claim 1, the second encodinggradient in the second direction further comprising at least one of adephasing gradient and a rephasing gradient.
 11. The method of claim 1,the distribution density of an undersampled k-space data set in thecenter region of the k-space being larger than the distribution densityof an undersampled k-space data set in the peripheral region of thek-space.
 12. The method of claim 1, the image reconstruction methodcomprising at least one of compressed sensing, parallel imagingtechnique, or partial Fourier reconstruction. 13-14. (canceled)
 15. Amagnetic resonance imaging (MRI) system comprising: an MRI scanner, acontrol unit, and a processing unit, the MRI scanner comprising: a mainmagnet field generator configured to generate a main magnetic fieldthrough a region of interest (ROI); a gradient magnet field generatorconfigured to apply a slice selection gradient to a slice of the ROI, togenerate a first encoding gradient in a first direction, and to generatea second encoding gradient in a second direction; and an RFtransmit/receive unit configured to transmit a plurality of RF pulses tothe slice of the ROI to generate a plurality of echoes, said gradientmagnet field generator being configured to apply the first encodinggradient in the first direction and the second encoding gradient in thesecond direction simultaneously on each echo, the amplitude of the firstencoding gradient when acquiring the center region of a k-space beinglower than that of the first encoding gradient when acquiring theperipheral region of the k-space; and the processing unit beingconfigured to generate a plurality of undersampled k-space data setsbased on the first encoding gradient in the first direction and thesecond encoding gradient in the second direction, and generate an MRimage by applying at least one image reconstruction method to theundersampled k-space data sets.
 16. The MRI system of claim 15, whereinthe plurality of RF pulses comprise fast spin echo (FSE).
 17. The MRIsystem of claim 15, wherein the waveform of the first encoding gradientin the first direction comprises three steady phases and two phases oftransition.
 18. The MRI system of claim 15, wherein the waveform of thefirst encoding gradient in the first direction comprises part of afunction having a smooth variation. 19-21. (canceled)
 22. The MRI systemof claim 15, wherein the second encoding gradient in the seconddirection comprises an oscillating waveform.
 23. (canceled)
 24. The MRIsystem of claim 15, the applying a first encoding gradient in the firstdirection comprising: applying at least two different encoding gradientsfor two different echoes, respectively.
 25. The MRI system of claim 15,the first encoding gradient in the first direction further comprising atleast one of a dephasing gradient and a rephasing gradient.
 26. The MRIsystem of claim 15, the second encoding gradient in the second directionfurther comprising at least one of a dephasing gradient and a rephasinggradient.
 27. The MRI system of claim 15, the distribution density of anundersampled k-space data set in the center region of the k-space islarger than the distribution density of an undersampled k-space data setin the peripheral region of the k-space.
 28. The MRI system of claim 15,the image reconstruction method comprising at least one of compressessensing, parallel imaging technique, or partial Fourier reconstruction.